In a typical photovoltaic system (subsequently “photovoltaic” will be abbreviated with “PV”, as is customary among experts), several PV modules are connected in series to form a PV string, in order to achieve a direct current voltage of several 100 to 1000 volts, in the future possibly even up to 1500 volts, that is appropriate for DC-AC conversion and subsequent network supply. Either the PV string is connected directly to a PV inverted rectifier (string inverter), or several strings having preferably the same structure and being exposed to similar irradiation conditions are connected in parallel and connected to a central inverted rectifier (central inverter). In order to reduce the wiring expenditure, the parallel circuit consisting of several strings can be combined to a PV sub-system, also called PV array, in a PV array combiner box (also called PV array box or PV string combiner). These, in turn, can be combined in PV generator combiner boxes which are finally connected to a central inverter, resulting in a tree structure in the power wiring.
FIG. 1 shows a PV system in the form of a one-PV power supply system according to DIN VDE 0100-712 (VDE 0100-712): 2006-06, image 712.1. It involves a PV system with only one PV generator. FIG. 2 shows a PV system according to DIN VDE 0100-712 (VDE 0100-712): 2006-06, image 712.2 with several PV arrays. Subsequently, its structure is described in more detail in the detailed description, whereas the basic structure is well-known among experts. The terminology used in this application corresponds to DIN VDE 0100-712 (VDE 0100-712): 2006-06, which is well-known among experts. However, for the sake of completeness, by reference the document is made subject of the present disclosure.
Increasing interconnection of the PV system reduces the possibilities of selectively recording and analyzing the operating data of sections of the PV system located upstream of the previously mentioned tree structure and, if required, taking steps for maintaining or optimizing the system.
One possibility for system control is that of integrating sensors in the PV array box (or PV string combiner). To this end, the voltage between the positive and negative pole, which is the same for each string, and selectively the currents of the individual strings are measured. As a result, it is possible to compare the power supplied from the individual strings—in this case, the measured values represent the average values of the superior string or in general the system part which is superior to the point of measurement and allow at best to a limited extent for a comparison of individual modules in the tree structure. PowerWiring interruptions and total failure of individual modules can be detected to some extent. However, if power losses occur in several modules assigned to different strings, for example, through several partially shaded strings or prematurely aging of individual modules, the possibility for detecting errors decreases because of a lack of reference values. Adjustments of string performance by means of reference points can improve the situation only to a limited extent because they are not exposed to the same ambient conditions, for example, temperature or wind.
The DE 40 32 459 C2 proposes to equip each module with an integrated inverted rectifier with MPP tracking. Typically, an MPP tracker adjusts the voltage to the point where the system operates at maximum power (maximum power point, MPP). For this purpose, the MPP tracker varies the current extracted by a small amount, calculates the power and adjusts the current value toward a higher output. By means of a control unit, signals are transmitted to a data bus which supplies these data to the power and control unit for the purpose of examining the functional capability of the module. This system has the disadvantage that it requires a multitude of inverted rectifiers with MPP trackers and has a rigid structure of data transmission.
The DE 102 22 621 A1 discloses a solar generator with a variable current bypass which is controlled in such a way that each generator is operated continuously in its respective actual, specific MPP. This does not allow for a complex system control. The DE 20 2007 011 806 U1 discloses a solar cell system with identification chips in which an individual identification code is stored for each solar cell.
The identification chips of the solar cells are connected with a central processor via a two-wire parallel bus and an interface circuit. The central processor comprises a processor memory in which all identification codes of the solar cell system are stored. The data is transmitted with identification numbers. Also this system has the disadvantage of having a rigid structure of data transmission and requiring an individualization of the solar cells.
The DE 20 2005 020 161 U1 proposes a device for monitoring photovoltaic panels in which in the dark residual voltage is collected in the generator combiner box and the measured values are supplied to a window comparator. The inverted rectifier has been provided with a central alarm device which has a decoder for giving a panel-related alarm when falling below a specific signal threshold.
Furthermore, the DE 198 59 732 A1 describes the transmission of information from the inverted rectifier to a center by time-division multiplexing transmitting the power via the power supply line. Unfortunately, this requires a disconnection from the supply line when the data are to be transmitted.
In each of the systems mentioned above the rigid structure of data transmission, the low flexibility, as well as the susceptibility to malfunction are of disadvantage, especially in large PV systems.